1. Field of the Invention
The present invention relates to an apparatus for filtering and degassing body fluids, in particular a blood filter, having at least two filter chambers, separated from one another by a filter medium, that are preferably arranged in a common housing, the first filter chamber being connected to an inflow connector for body fluid that is to be filtered, and the second filter chamber to an outflow connector for filtered body fluid.
2. Related Prior Art
An apparatus of this kind is known from DE 35 41 521 C2.
The known apparatus is a blood filter whose inflow connector, in the operating position, is connected vertically from below to the first filter chamber. The inflow connector leads into a rising tube that passes through the cylindrical first filter chamber to approximately two-thirds of its height. The blood being filtered emerges at the top from the rising tube, which is configured as an overflow, and falls along the outside of the rising tube into a sump.
As these overflows occur, the gas dissolved in the blood is separated out in the form of gas bubbles that rise upward. At the top of the first filter chamber is a gas outlet through which the first filter chamber is vented.
The rising tube is surrounded by a hollow-cylindrical filter medium adjoined externally by a hollow-cylindrical second filter chamber that is equipped at its upper end with a gas outlet opening for venting the second filter chamber.
In the operating position, the second filter chamber is connected at the bottom to a horizontally proceeding outflow connector through which the filtered blood emerges from the known blood filter.
The known blood filter is used for extracorporeal blood circulation systems, in which blood is filtered and purified of gas bubbles before being returned to the patient. This step must not damage the blood cells or trigger any coagulation mechanisms. Blood losses must also be avoided. In the known filter, the separation of particles and aggregates is performed in the form of a dead-end filtration system, although this has the disadvantage that the retained constituents gradually accumulate in the filter. In the case of the known filter, these constituents collect in the sump, as a result of which the filter medium gradually becomes clogged, progressively from bottom to top, by sedimenting particles and aggregates.
In the case of a blood filter incorporated into an extracorporeal blood circulation system, this clogging can result in an undesirable reduction in the volumetric flow of the filtered whole blood, or even in complete interruption of the circulation system and an impermissible rise in pressure across the filter, which has an undesirable effect on the blood constituents.
It is known for this purpose that in the case of a blood filter of the aforesaid kind, a bypass having, for example, two Y-pieces is constructed, the two arms of the two Y-pieces being connected to the blood filter and to a bypass line, respectively the respective upright of the Y-line then forms the inflow or the outflow for the parallel arrangement made up of blood filter and bypass. A shutoff valve is incorporated into the bypass line, although it is also known to interrupt the bypass line using a tubing clamp.
If the blood filter has become impermissibly clogged, the bypass line is opened so that blood circulation can be maintained around the blood filter.
This specific arrangement of the blood filter with bypass line requires greater complexity in terms of equipment as well as special cleaning actions, and is therefore disadvantageous. A further disadvantage with this bypass line is the fact that coagulated blood can collect in the bypass line above the shutoff valve or tubing clamp and, when the bypass line is suddenly opened, can pass into the blood circulation system in the form of a plug and, in unfavorable cases, result in embolisms.
A further disadvantage with the known blood filter is the fact that the two separate vent openings create the need for iterative interventions in order to vent the blood filter completely. Venting of this kind is, however, necessary prior to use of the blood filter in the extracorporeal blood circulation system in order to prevent gas bubbles from getting into the blood circulation system and causing known negative effects.
With the known blood filter, therefore, there exist considerable handling and sterility problems that are associated with the bypass line and with the two venting openings. In addition, the known blood filter exhibits safety risks in use, resulting both from the bypass line and from sedimentation of the separated particles and aggregates.
An additional general disadvantage with the known blood filter is its bulky construction, which results in particular from the fact that the venting space of the inner, first filter chamber is arranged above the rising tube and the second filter chamber.
DE 196 20 591 discloses a more compact apparatus for removing gases from liquids and using the principle of degassing by overflow, known from the blood filter cited above. In this case the overflow connection is provided between a cylindrical inner and a hollow-cylindrical outer filter chamber, the flow space that guarantees the overflow being equipped with a hydrophobic filter that discharges the emerging gas upward but holds back the blood. With this apparatus the blood itself is not passed through a filter medium, so this is therefore not a blood filter.
A further, very special and not universally applicable filter for hemofiltration or plasmapheresis, i.e. in which there is a large pressure drop across the filter medium, is known from EP 0 076 421. This filter comprises an inner and an outer fiber bundle, connected respectively to an inlet connection and outlet connection. There is also a filtrate outlet; in other words this is not a dead-end filter but rather a crossflow filter, with a corresponding blood loss.
Arranged between the inlet and the outlet is a kind of pressure relief valve that automatically creates a connection between the inlet and outlet if the pressure drop across the series-connected fiber bundles becomes too great.
For safety reasons, this filter is of only limited suitability for use in extracorporeal blood circulation systems. One disadvantage of the filter is the fact that activation of the pressure relief valve is not visible, so that operating personnel cannot detect whether the filter is clogged or is still working satisfactorily. In addition, the pressure relief valve creates the risk of inadvertent opening and of leakage flows. The reason is that in specific pressure ranges, it opens only partially; and the opening pressure cannot be established reproducibly because of aging and stress processes and fluctuations in the properties of the materials used.
A further disadvantage of this filter is that it "cycles," i.e. after opening in response to an impermissibly high pressure rise across the filter medium, that pressure then usually drops again, so that the pressure relief valve closes again; this then in turn results in a pressure buildup that opens the pressure relief valve again.